I picked up a used but never fired 11-87 Police model. It runs GREAT on everything except low brass birdshot (1.125 oz @ <1200fps). However, when I clean and re-assemble it everything is nice and tight but after a few rounds at the range, it develops a slight wiggle between the barrel and the receiver. Not front to back but side to side and very slight. The plastic forend moves side to side slightly as well. The end cap is snug and does not loosen up at all. Is this slight amount of side to side play between the barrel and the receiver anything to worry about?

BTW- I really like this gun! It points naturally and the heaviest Buck and Slug loads feel like I am shooting skeet.

Note that the magazine extension is TWO pieces - the tube and the nut. Separate the parts, screw the nut on first to get a nice snug fit (I think there is a ball detent), then screw on the tube and secure with the clamp. The clamp is mandatory.

Yep, I screw on the end cap THEN the mag extension and then the clamp. It still develops a very slight wiggle (but the end nut does not loosen)

any chance the barrel alignment pin on the left side of barrel extension is sheared?

That is what I wanted to post

No, it is still there as well as the barrel groove and the stud on the inside of the receiver, but thanks for the responses.

A detail that may or may not be significant;

My gun has Remington factory synthetic furniture AND a gas cylinder collar. In passing, someone told me today that they had never seen the gas cylinder collar equipped on an 11-87P with a synthetic forend. I am 99.99% certain that my gun came from the factory set-up like this. Is it possible that the combination of the synthetic forend and gas cylinder collar has created too much play in the barrel to receiver connection?

The gun, while in new condition, is an older model than current offerings as it has a raised comb, molded plastic buttstock and protective ears on the rear aperture sight.

That was it!

As an experiment, I removed the Gas Cylinder Collar and re-assembled everything else. Immediately, the gun felt tighter and after 20 rounds of heavy 00 Buck (9 pellets @ 1300 fps) everything is still tight as a drum.

A google search reveals that there is a lot of ambiguity regarding what guns NEED the GCC and what guns DON'T need this part whose purpose is equally speculative. Apparently Remington is confused as well since for at least a period of time they were shipping seemingly identical guns, some with and some without the GCC. Also, their Operators Manual is somewhat less than unhelpful

PSA- My factory 11-87P did not need this part and seems to operate better without it. YOUR 11-87 MAY  need it depending on a a few factors.